1. Field of the Invention
The present invention relates to plunger operated pipets for picking up and dispensing predetermined volumes of fluid and, more particularly, to pipetes incorporating mechanisms for controlling the extent of plunger movement during successive fluid pickup and dispensing operations.
2. Description of the Prior Art
Pipets are widely used in laboratory and clinical procedures which require a predetermined fluid volume to be aspirated from one vessel and the same or a different fluid volume to be dispensed into another vessel. Such pipets typically include a plunger or piston which is actuated in one direction to aspirate or draw fluid into an open end of the pipet and in the opposite direction to discharge fluid from the open end. In a common form, such pipets are sized to fit in an operator's hand and include a plunger actuator controllled by the operator's thumb or finger to drive the plunger in one direction and a return spring to drive the plunger in the opposite direction. The operator depresses the plunger actuator to drive the plunger forwardly against the spring force and expel air from the open end. The open end then is immersed in fluid and the plunger retracted rearwardly by the spring drawing fluid into the open end. Next, the pipet is positioned over or in a receiving vessel, and the plunger is again driven forwardly to expel fluid from the pipet into the vessel. The pipet is withdrawn from the vessel and the plunger retracted rearwardly to prepare for the next pickup operation. Usual pipet action thus comprises two successive plunger strokes--a fluid pickup stroke followed by a fluid dispensing stroke--each stroke comprising plunger movement in foward and rearward directions. In some laboratory procedures, the pipet tip is immersed in fluid (e.g. reagent) in the receiving vessel, and the fluid dispensing stroke comprises repeated actuation of the plunger back and forth to thoroughly mix the reagent and the dispensed fluid by the turbulent intake and discharge of both.
Fluid contamination of the operating pipet elements is typically avoided by means of a replaceable, conical plastic tip fitted on and sealed around the open end of the pipet. The available interior volume of the tip is larger than the fluid pickup capacity of the pipet. As a result, plunger actuation draws fluid only into the tip and thereafter dispenses the fluid from the tip. After the dispensing stroke, the operator removes and disposes of the tip and replaces it with a new tip. As a result the pipeted fluid does not contact and hence cannot contaminate other elements of the pipet. Moreover, no fluid remains to contact and thus contaminate the next fluid sample picked up.
In some laboratory procedures it is desired that the pipet plunger be capable of executing different length strokes of plunger movement in succession. For example, often a relatively short pickup stroke is to be followed by a longer discharge stroke. In this manner the total volume of fluid picked up is discharged from the pipet followed by a small slug or volume of air. The scrubbing action of the air slug as it leaves the tip helps to expel any residue of fluid which might otherwise cling to the tip. When minute microliter sample volumes are being dispensed, any fluid residue which remains on the pipet tip, even as little as one drop, can represent a significant fraction of the dispensed fluid volume and hence can lead to large measurement errors in the laboratory procedures or analyses being performed. The longer dispensing stroke helps to avoid such errors.
On the other hand, in other procedures it is desired to pick up with a long stroke and dispense with a shorter stroke so that some of the fluid picked up remains in the tip after dispensing is complete. For example when pipetting fluid into a nephelometric analysis cell which measures the extent to which light is scattered by the cell contents, wide fluctuations and hence corresponding errors in measured light scatter are caused by air bubbles present in the cell. By dispensing fluid into the nephelometric cell using a shorter plunger stroke some fluid will remain in the pipet tip at the conclusion of the dispensing stroke and, therefore, no interferring air bubbles will be discharged from the pipet into the nephelometric cell.
U.S. Pat. Nos. 3,506,164 and 3,766,784 illustrate prior pipet arrangements for executing successive plunger pickup and dispensing strokes of different lengths. Each patent illustrates an internal indexing mechanism responsive to forward and rearward plunger movement causing the pipet plunger to execute an alternating stroke sequence of long-short-long-short-long . . . , etc., ad infinitum. That is, plunger actuation always causes the plunger stroke length to alternate back and forth between two different stroke lengths so that each plunger stroke is always different in length than the preceding one.
While the aforementioned patented devices are often used in the above-described pipetting operations, they exhibit a number of operational drawbacks which reduce their overall attractiveness. A first drawback is that an operator upon observing or holding the pipet cannot be certain whether the next stroke will be long or short. As a result, to be certain, the operator must depress the plunger actuator one or more times, observe the alternating plunger stroke sequence, and terminate the plunger action after a short stroke if long pickup stroke is desired or vice versa--all simply to ensure that the stroke will be either long or short as desired. Unless the operator accurately checks the stroke length sequence before each pipet operation, it is possible that the wrong sequence will be executed thereby causing a major error in the volume of fluid dispensed and measured. A second drawback of the aforementioned devices results from the fact that the indexing mechanism for executing the alternating stroke length sequence receives and operates in response to the axially directed actuating force applied to the plunger by the operator. As a result, the operating indexing elements of the pipets are subject to force loading causing the elements to wear and hence increasing the likelihood of premature part failure. Moreover, they are subject to excessive force loading from operator misuse or abuse increasing the probability of jamming or other malfunctioning of the operating elements.
As a result, there is a need for a pipet plunger stroke length control mechanism which does not require operator attention to check the stroke sequence and which is less susceptible to jamming or other mechanical failure.